The present invention relates to the field of geophysical seismic operations for evaluating subsurface geologic formations. More particularly, the invention relates to an integrated marine acoustic energy source and method for discharging source energy into subsurface geologic formations.
Seismic streamer cables are towed in the water behind a marine seismic vessel to detect seismic source energy reflected from geologic features. The vessel tows air guns or other acoustic energy sources to generate energy propagating downwardly through the water and subsurface geologic formations. A portion of the energy is reflected by interfaces between subsurface formations and is detected with hydrophones attached to the streamers. The streamers typically comprise hydrophone strings, internal electrical conductors and control wires, and buoyancy material. Typical streamer cables are three to eight kilometers in length and are towed below the water surface to avoid acoustic noise and vibration generated by surface wave action and other environmental factors.
Various marine vibrators and other acoustic energy sources are used in marine seismic surveys to generate acoustic source energy. Conventional acoustic energy sources are cylindrical, bell-shaped, or spherical in shape and are negatively buoyant in water. The hydrodynamic drag coefficient is high for the acoustic energy sources and for the umbilicals providing compressed air and other support to the acoustic energy sources. It is not uncommon for air-gun sources and associated support equipment to produce more than 20,000 pounds of drag. Accordingly, a seismic vessel expends significant energy in towing conventional systems through the water.
The data detected by receivers such as hydrophones is recorded and processed to provide information about subsurface geologic formations. In an effort to improve seismic signal resolution, various systems use multiple acoustic sources such as air guns in one or more seismic arrays. For example, U.S. Pat. No. 4,727,956 to Huizer (1988) disclosed a method of arranging a plurality of seismic sources in a seismic array having at least two subarrays. The sources were operated to provide a predetermined relation comprising a function of the operating pressure, air gun volume, and tow depth. The seismic source array was modified by changing operating parameters such as the primary-to-bubble ratio and signature shape for the signals.
U.S. Pat. No. 3,953,826 to Brundit et al. (1976) disclosed a means for controlling the streamer cable length in an array having multiple sources and arrays. The multiple arrays were towed in a straight line behind the vessel. In U.S. Pat. No. 4,323,989 to Huckabee (1982), at least two arrays of seismic sources were towed parallel to the other. Steering devices maintained each streamer in the desired lateral position.
Multiple seismic sources require power and control systems for activating the seismic sources. For air gun systems, compressed air hoses and control wires link subsystem components on the seismic vessel to each air gun. One disadvantage of such systems is that the compressed air hoses add weight and tow resistance to a seismic system. Additionally, the discharge of multiple impulsive acoustic energy sources imparts significant acoustic energy to water which interferes with proximate seismic recording operations, and is criticized as having potential environmental ramifications. Accordingly, a need exists for an improved system which effectively generates acoustic source energy in marine seismic operations, reduces hydrodynamic drag, improves signal noise characteristics, and reduces environmental impacts.
The invention provides a system and method for generating acoustic source seismic energy in water. The system comprises at least two streamers, an electrical conductor within each streamer, and a plurality of housings disposed within each streamer at selected positions along each streamer. A plurality of electrically activatable acoustic energy sources are each engaged with a housing and with an electrical conductor for receiving electric power and for selective operation to generate acoustic source energy in the water, and a controller is engaged with each acoustic energy source for controlling operation of each acoustic energy source.
In different embodiments of the invention, each acoustic source can comprise a slotted cylinder moveable with one or more piezoelectric elements. The controller can be operable to activate the acoustic energy sources in a way to generate acoustic source energy detectable as zero-offset source-receiver locations and is further operable to produce bi-static reflections.
The method of the invention comprises the steps of deploying at least two streamers in the water, wherein each streamer includes an electrical conductor and a plurality of electrically activatable acoustic energy sources integrated within each streamer at selected positions, of providing electricity to each acoustic energy source through the electrical conductor, and of operating the controller to selectively activate at least one acoustic energy source to generate acoustic source energy in the water.